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Seed Conservation Research: IPGRI's Strategies and Activities

Volume 2 Number 9 - December 1997

H. Fassil & J Engels

I. Introduction

Seeds are the most convenient form in which to store and distribute plant germplasm. Genebanks in which dried seed is stored at low temperature are the most widespread form of ex situ conservation. Seeds which can be stored in this manner are referred to as "orthodox" (Roberts, 1973) and many species, particularly major food plants, produce this category of desiccation-tolerant seed. Over the years, techniques for conserving orthodox seeds for several decades have been developed. These involve drying seed to low moisture content (mc) (3-7% fresh weight, depending on the species) and storing them, in hermetically-sealed containers, at low temperature, preferably at -18ºC or cooler (FAO/IPGRI, 1994). These procedures have been widely adopted by seedbanks worldwide.

However, many species of tropical or subtropical origin have seeds which are sensitive to drying and chilling and cannot be stored in conventional genebanks. Seeds of this type are termed "recalcitrant" and even when stored under optimal conditions, their lifespan is limited to a few weeks, occasionally months. Recent investigations have identified species exhibiting an "intermediate" storage behaviour. While such seeds can tolerate desiccation to fairly low moisture content, once dried, they become particularly susceptible to injury caused by low temperature (Ellis et al., 1990). In comparison with truly recalcitrant seeds, the storage life of these intermediate seeds can be prolonged by some drying, but it remains impossible to achieve the long-term conservation which can be realized for orthodox seeds.

Numerous botanic gardens worldwide maintain seedbanks, mainly as short-term active collections for distribution and exchange purposes. Many have also incorporated long-term seed storage as an integral part of their operations. Botanic gardens generally tend to focus on the conservation of plant genetic diversity at the species level. On the other hand, conventional crop seed genebanks which have historically been linked to crop germplasm enhancement efforts, have been primarily concerned with intraspecific diversity. Despite this difference and although much of seed conservation research has typically been geared towards the specific needs of conventional crop plant seedbanks, the relevant knowledge and experience acquired to date are nevertheless of interest to botanic gardens that maintain seed collections.

II. IPGRI's Seed Conservation Research Strategy

In the past, the development of methodologies and procedures to store orthodox seeds was a major focus of IPGRI's seed conservation research. Improving the established techniques and making them more widely applicable remains a major part of the Institute's research agenda.

IPGRI's overall strategy in supporting research related to seed conservation is based on its four institutional objectives:

strengthening national plant genetic resources (PGR) programmes

contributing to international collaboration in the conservation and use of PGR

In pursuit of these objectives, IPGRI works in partnership with the broader scientific community at large. Although it undertakes no "hands-on" research of its own, the Institute aims to contribute to the relevant scientific processes through the identification and prioritisation of problems and opportunities; formulation of research designs; interpretation and synthesis of research results as well as their wide dissemination and application, and providing relevant training programmes. By bringing this modus operandi to the area of seed research, IPGRI aims to complement ongoing efforts in seed conservation research worldwide.

Over the years, the Institute has developed strong collaborative partnerships with several research centres including the Seed Laboratory of the University of Reading (U.K.), Royal Botanic Gardens, Kew (U.K.), the Boyce Thompson Institute (U.S.A.), Cornell University (U.S.A.), the National Seed Science Laboratory at Fort Collins (U.S.A.), the University of Wageningen (The Netherlands) and the Beijing Botanical Garden, Institute of Botany, Chinese Academy of Sciences (China). The recommended standards for orthodox seed conservation, for example, are based principally on research carried out at Reading with IPGRI support. In addition, a set of benchmark handbooks on seed technologies for genebanks were produced in conjunction with Reading and other collaborators (see Box 1).

IPGRI's current priorities in seed conservation research reflect the emphasis the Institute now gives to addressing the conservation problems of genetic resources programmes of developing countries. Collaborative research activities, involving institutes in both developing as well as developed countries are an integral part of this strategy. The rise in the number of genetic resources programmes over the last 20 years has principally been in the developing world, which is situated mainly in the tropics. Hot and humid tropical environments are particularly unfavourable for prolonging the longevity of orthodox seeds. The resulting problems include not only the rapid loss of viability which occurs under high humidity and temperature levels, but also the difficulties and costs involved in running conventional genebank operating dehumidification and refrigeration equipment under these climatic conditions. Furthermore, many of the "problem" species with recalcitrant seeds are predominantly of tropical origin. Accordingly, IPGRI has focused its seed conservation research efforts on the following two aspects:

to advance understanding of the nature of desiccation sensitivity and develop ways of overcoming it

to develop low-input techniques to dry and store orthodox seeds

These objectives are pursued through a combination of three complementary approaches:

implementing collaborative research projects (involving partners in both developing and developed countries)

stimulating and encouraging genebanks and researchers to conduct (applied) research on relevant aspects of seed conservation, especially on locally important plant genetic resources

promoting accessibility of relevant information

1. Collaborative Research Projects

International collaboration in research is essential, not only to ensure wider applicability, but also to facilitate the transfer of relevant technologies and to strengthen research capacity. Participation of developing countries in such research is vital as these countries are custodians of the major component of plant genetic diversity. Frequently, scientists in these countries have an intimate knowledge of many of the species. Furthermore, the development of techniques suited to the difficult and often resource-limited circumstances of many developing countries can only be effectively undertaken with the involvement of local institutions in such important applied and adaptive research. Many developed-country institutes, on the other hand, have the facilities and expertise for carrying out complementary research work on the more fundamental research questions.

2. Stimulating Applied Research : Transforming research results into practical procedures and options

IPGRI has initiated a series of technical bulletins which will focus on important management aspects in genebanks. In addition to providing an overview of the state-of-the-art in a simple and understandable manner, technical bulletins are intended to contribute ideas and suggestions for conducting research on "new" species and/or to present options for applying existing procedures to local conditions. The bulletins aim not only to encourage research initiatives among genebank workers world-wide but also to provide a clearer focus for such efforts. The first volume entitled A protocol to determine storage behaviour is indicative of the specific approach to be taken in this publication series . Other technical bulletins being prepared on various subjects include the principles and procedures of seed drying; seed moisture content testing; seed treatment; seed germination procedures, and low-input cryopreservation procedures.

3 Promoting Accessibility of Relevant Information

As plant genetic resources programmes have increased in number and expanded in scope, so the range of species requiring ex situ conservation has broadened from major crops to include forestry species, and wild and underutilized species. As information on the seed storage behaviour of many of these species is scattered in the literature, IPGRI in collaboration with the University of Reading and the Royal Botanic Gardens at Kew, U.K. produced a Compendium of seed storage behaviour (see Box 3). The Compendium, which complements the IPGRI Technical Bulletin series (e.g. The protocol for the determination of seed storage behaviour) provides species-specific information on the seed storage behaviour of 7,155 species (Hong et al., 1996). In the near future, this information will be made available in electronic form as a searchable database. Work is also underway to add information on seed germination procedures and dormancy to this database and it is anticipated that availability of future technical bulletins in this field will stimulate research and generate more information on additional species.

III. Ongoing Research Activities Supported by IPGRI

IPGRI supports a number of activities by employing a combination of these approaches. Following are some specific examples.

Ultra-dry Seed Storage

Maintaining seed germplasm in cold storage is still problematic for many genebanks. Lack of proper refrigeration equipment, unreliable electricity supply, poor maintenance practices and high operating costs are some of the key constraints. Many genebanks, especially those in developing countries, lack the resources and facilities to carry out their tasks effectively. The "ultra-dry" method of seed storage, which involves the maintenance of very dry seeds under ambient or partly cooled conditions, may provide a cost-effective alternative for such resource-constrained genebanks.

Accordingly, research on lowering seed moisture content continues to be an important part of IPGRI's seed research agenda. IPGRI initiated a project on ultra-dry seed storage in 1989, in cooperation with the University of Reading, U.K., the Genetic Resources Unit, Horticulture Research International, Wellesbourne, U.K., and the Centre for Plant Breeding and Reproduction Research (CPRO-DLO), Wageningen, the Netherlands. Results obtained for the species included in this 5-year study, namely carrot, groundnut, lettuce, oilseed rape and onion, confirm that at 20ºC ultra-dry storage leads to better survival than storage of seed at higher moisture contents. It was also observed, however, that storage at -20ºC rather than 20ºC was beneficial to seed survival (Ellis et al., 1996). However, significant differences have been reported with regard to the amenability of different species to this ultra-dry technology, optimum seed moisture contents with respect to storage temperature and the occurrence of a lower limit to moisture content below which seed can be damaged by further drying.

With a view to reconciling these different findings, a collaborative five-year research project involving the National Seed Science Laboratory at Fort Collins (U.S.A.), the International Crops Research Institute for the Semi-Arid Tropics (ICRISAT) at Hyderabad, India and the Institute of Crop Germplasm Resources of the Chinese Academy of Agricultural Sciences (CAAS), Beijing, China, is presently investigating several aspects related to the interaction between storage temperature and optimal seed moisture content using lettuce seed as a model. This international effort is being coordinated by IPGRI, in close consultation with the University of Reading, U.K.

Addressing the conservation problems of intermediate and recalcitrant species has again increased in importance with the addition of forestry genetic resources to IPGRI's programme. IPGRI is currently coordinating a project to study and improve the handling and storage of intermediate and recalcitrant forestry species. Thirteen institutions in developing countries, predominantly forest research institutes, and ten research institutions in Europe and the USA participate in this research network. At present, 24 selected tropical tree species are being studied according to an agreed research protocol (Ou‚raogo et al., 1996).

Sun-drying: an Alternative Way to Lower Seed Moisture Content

Another collaborative research activity involving IPGRI, the National Genebank of Kenya, the KARI Mtwapa Agricultural Research Centre and the Royal Botanic Gardens, Kew, U.K. is investigating the effects of sun-drying on seed quality. Investigations will include viability and ageing tests as well as seed health testing. The experiments are being conducted on millet, maize and groundnuts and comparisons are to be made with drying under shade and in a dehumidified drying room.

Cryopreservation: a Potential Alternative for Conserving Seed of "Problem" Species

In the case of recalcitrant and intermediate seed types, research should be aimed at developing ways for manipulating or treating seeds or embryos in order to induce desiccation and chilling tolerance. Development of such methods could enable storage of recalcitrant and intermediate seed following conventional procedures used for orthodox seed. They could also aid efforts to improve the survival of excised embryos after cryopreservation. Optimizing seed drying methods, in particular with respect to the rate of seed desiccation is one important aspect requiring investigation.

Currently, IPGRI supports research in this priority area in South Africa, through a project which involves screening seeds and excised embryos of various tropical tree species for their tolerance to desiccation and freezing. Thus far, it has been demonstrated that seeds of Carica papaya and Azadirachta indica can withstand cryopreservation after desiccation down to very low levels of moisture content. In the case of Camellia sinensis, Trichilia dregeana and Ekebergia capensis, however, only excised embryos were found to survive freezing after partial dehydration (Berjak, 1996).

Germplasm Health: a Vital Consideration

International movement of germplasm entails the risk of spreading pests and diseases to new areas. Germplasm health measures are important at all stages of plant genetic resources management: collecting, multiplication, characterization, storage, conservation, and to all forms of plant germplasm - be it seed or other plant tissue. It is a particularly important consideration during international exchange of plant material. Through the production of a series of crop-specific technical guidelines, the joint programme between FAO and IPGRI aims to provide relevant information on disease indexing and other procedures which will help to ensure phytosanitary safety when germplasm is exchanged internationally. The Guidelines reflect the consensus of the eminent specialists in the field on the significance of pests and pathogens and recommended measures for their detection and control.

The FAO/IPGRI Technical Guidelines for the Safe Movement of Germplasm are important for all those engaged in wide distribution of plant material, including botanic gardens. To date, volumes on the following species, which include both seed and vegetatively propagated plants, have been published: cassava, Citrus, cocoa, coconut, edible aroids, Eucalyptus, grapevine, legumes, Musa, small fruits, small grain temperate cereals, sugarcane, sweet potato, stone fruits, Vanilla and yam.

The conservation and use of genetic resources involves many stakeholders and requires a wide range of approaches and solutions. It is essential in all areas of plant genetic resources work that the various actors involved effectively complement each other. Botanic gardens around the world have an important role to play in this endeavour. Most of the world's ca. 1,500 botanic gardens conserve wild relatives of cultivated crops, medicinal and forestry species as well as a diversity of ornamental plants (FAO, 1996). Many maintain collections of threatened and/or rare plants as well as minor/underutilized crops. Such species are often conspicuously absent in other ex situ collections. Botanic gardens can therefore fill this important gap. In just the way that practical experience and knowledge of conventional seed banks can be useful for botanic gardens, so seedbanks stand to benefit greatly from species-specific information including the basic research related to taxonomy and plant biology carried out by many botanic gardens. The FAO report on the State of the World's Genetic Resources noted that linkages between botanic gardens and crop-oriented genebanks are weak (FAO, 1996). By bringing together a wide range of partners with different priorities, expertise and comparative advantages, IPGRI works to strengthen such collaboration with the aim to ensure the safe conservation and sustainable use of plant genetic resources.